When digitising images, such as photographs, it is desirable to be able to automatically measure the size of the object being imaged so that the image may be reproduced to scale. When a camera is used to capture images of objects of different sizes, then an operator will often zoom in on the image, by varying the camera lens focal length, so that the object image fills as much of the viewfinder as possible. This allows for greater reproduction of picture quality because more image information is captured at source. It means, however, that the actual size of the object is independent of its size in captured image coordinates, pixels. To measure such an object accurately, it would normally be necessary to know the focal length of the lens, to determine image magnification and thus calculate the size of the object in absolute coordinates (millimeters).
Use of reticles to assist in determining the size of objects under magnification is well known. It is common to see a printed scale superimposed on images to enable a viewer to determine the size of an object, for example, a transparent ruler. Printed scales, however, are of little use in enabling a computer to determine the absolute size of an object being imaged when the magnification achieved by a zoom lens camera for each image is an unknown variable.
It is an object of the present invention to mitigate such problems.